1. Field of the Invention
The present invention relates to an optical waveguide used mainly, e.g., in optical communications and a method for manufacturing the optical waveguide.
2. Description of the Related Art
With the progress in the market of optical communications, optical components need to achieve both high performance and low cost. In particular, there has been an increasing demand for low-cost passive optical components that do not operate themselves.
The fabrication of an optical waveguide requires a very fine precise pattern. The specification of pattern accuracy is strict particularly for a single mode optical waveguide. A general method for forming such a pattern is dry etching, which has been used often in a semiconductor process. The conventional manufacturing process of a single mode optical waveguide for optical communications will be described below by referring to the drawings.
FIGS. 12A and 12B show the configuration of a general single mode silica glass optical waveguide. FIG. 12A is a plan view of the optical waveguide, and FIG. 12B is a cross-sectional view taken along the line A—A in FIG. 12A. A core 121 that serves as a waveguide layer is formed in a cladding 122. The refractive index of the core 121 is higher than that of the cladding 122. Light under certain conditions is trapped and propagated in the core 121 in the direction of the arrow 123. For example, when a guided optical wave having a wavelength of 1.3 μm to 1.55 μm, the core 121 generally is about 8 μm square in cross section, as shown in FIG. 12B. The core 121 can be patterned into a desired shape, e.g., Y-branch, thereby providing various optical circuit structures. The shape and surface roughness of the core significantly affect the light propagation ability.
FIGS. 13A to 13C show the process of a general method for manufacturing a conventional silica glass optical waveguide. First, a core film 131 is formed on a quartz substrate, which is also used as a lower cladding layer 132, by flame hydrolysis deposition (FHD), as shown in FIG. 13A. In the FHD process, a flame of H2 and O2 is produced in the air, and SiCl4 and a small amount of GeCl4 are mixed and hydrolyzed in the flame to form Ge-doped SiO2 (a core film 131). The resultant SiO2 is deposited on the quartz substrate in fine powder form, heated at temperatures of not less than 1000° C., and thus changed to glass. The glass SiO2 is the core film 131 When a substrate other than the quartz substrate is used, the lower gladding layer 132 should be formed on this substrate by FHD before forming the core film 131.
Next, the core film 131 (FIG. 13A) is patterned into a desired shape by photolithography and dry etching, resulting in a core 131a (FIG. 13B).
Further, an upper cladding layer 133 is formed on the lower cladding 132 and the core 131a by FHD (FIG. 13C). An optical waveguide thus produced can achieve low loss and good characteristics.
In addition to the quartz material, resin has been studied recently as an optical waveguide material. At present, resin is inferior to quartz in both transmission capacity and reliability. However, resin can be molded easily compared with quartz and exhibit high transmission capacity for light in the wavelength region of 650 nm to 850 nm. Therefore, resin is a very promising material for an optical waveguide. Examples of the resin material include polymethyl methacrylate (PMMA) having excellent transparency. A resin material obtained by deuteration or fluorination of acrylic resin, epoxy resin or polyimide resin also has been used in recent years. This resin material absorbs less light in the wavelength region of 1.3 μm to 1.55 μm. Accordingly, the above materials can provide a low-loss optical waveguide.
A general manufacturing method for an optical waveguide using a resin material includes forming a core layer and a cladding layer mainly by spin coating and patterning the core layer by dry etching.
As described above, whether quartz or resin, the conventional method has to repeat the deposition of the cladding layer that has a thickness of not less than 20 μm. Then, the core layer is formed and patterned into a convex shape by dry etching. However, complicated equipment is required to perform the dry etching. Therefore, the conventional method has the problems of cost and productivity. To solve the problems, various methods for manufacturing an optical waveguide have been proposed. A typical example of those methods is a groove-filling technique.
An example of an optical waveguide with a filled groove is disclosed in JP 63(1988)-139304 A, JP 8(1996)-320420 A, or JP 11(1999)-305055 A. The groove-filling technique will be described by referring to FIGS. 14A to 14D, which show the process of a method for manufacturing an optical waveguide with a filled groove.
As shown in FIG. 14A, a groove 142 that corresponds to a desired core pattern is formed in a cladding 141 (a glass or resin substrate). In this case, dry etching can be used generally to form the groove 142. Then, the groove 142 is filled with a core material 143 whose refractive index is higher than that of the cladding 141 (FIG. 14B). The overflow 143b from the groove is removed, and a core 143a is formed in the substrate 141 (FIG. 14C). Finally, a cladding 144 is formed on the core 143a and the substrate 141 (FIG. 14D), thus producing an optical waveguide with a filled groove. Although this method is similar to that shown in FIGS. 13A to 13C in the use of dry etching, it can achieve higher efficiency and productivity than the method shown in FIGS. 13A to 13C.
However, the groove-filling technique causes different problems when a quartz material is used as the core material and when a resin material, typified by acrylic resin, epoxy resin, or polyimide resin, is used as the core material.
The following is an explanation of the problem of a quartz glass material. Typical examples of a method for filling the core material in the groove of an optical waveguide include FHD, CVD, vacuum deposition, and sputtering. For a single mode optical waveguide, the core should have a thickness of about 8 μm. For a multimode optical waveguide, the core should have a thickness of as much as several tens μm. It takes a considerable length of time to form such a thick film, which results in a production disadvantage.
The following is an explanation of the problem of a resin material such as acrylic resin, epoxy resin, and polyimide resin. When a resin material is used as the core material, the necessary film thickness can be achieved easily, e.g., by spin coating. However, the removal of the overflow 143b as shown in FIG. 14C is a problem. The resin material has low hardness, so that small flaws are generated on the surface of the core 143a due to polishing. These flaws cause the scattering of a guided optical wave and leads to a large waveguide loss. As an alternative method, dry etching can be used to remove the overflow. However, the dry etching has the disadvantage of cost as described above.
Therefore, even if an optical waveguide is produced by the groove-filling technique that uses a quartz material or a resin material such as acrylic resin, epoxy resin, and polyimide resin, the optical waveguide cannot achieve high productivity and high performance.